A Synchronized Class Example

The class, SynchronizedRGB, defines objects that represent colors. Each object represents the color as three integers that stand for primary color values and a string that gives the name of the color.

public class SynchronizedRGB {
 // Values must be between 0 and 255.
 private int red;
 private int green;
 private int blue;
 private String name;
 private void check(int red,
 int green,
 int blue) {
 if (red < 0 || red > 255
 || green < 0 || green > 255
 || blue < 0 || blue > 255) {
 throw new IllegalArgumentException();
 }
 }
 public SynchronizedRGB(int red,
 int green,
 int blue,
 String name) {
 check(red, green, blue);
 this.red = red;
 this.green = green;
 this.blue = blue;
 this.name = name;
 }
 public void set(int red,
 int green,
 int blue,
 String name) {
 check(red, green, blue);
 synchronized (this) {
 this.red = red;
 this.green = green;
 this.blue = blue;
 this.name = name;
 }
 }
 public synchronized int getRGB() {
 return ((red << 16) | (green << 8) | blue);
 }
 public synchronized String getName() {
 return name;
 }
 public synchronized void invert() {
 red = 255 - red;
 green = 255 - green;
 blue = 255 - blue;
 name = "Inverse of " + name;
 }
}

SynchronizedRGB must be used carefully to avoid being seen in an inconsistent state. Suppose, for example, a thread executes the following code:

SynchronizedRGB color =
 new SynchronizedRGB(0, 0, 0, "Pitch Black");
...
int myColorInt = color.getRGB(); //Statement 1
String myColorName = color.getName(); //Statement 2

If another thread invokes color.set after Statement 1 but before Statement 2, the value of myColorInt won't match the value of myColorName. To avoid this outcome, the two statements must be bound together:

synchronized (color) {
 int myColorInt = color.getRGB();
 String myColorName = color.getName();
} 

This kind of inconsistency is only possible for mutable objects - it will not be an issue for the immutable version of SynchronizedRGB.